1. Field of the Invention
The present invention relates to a decoder for composite video signals, which is used to separate a luminance signal (Y) and a color signal (C) from a composite video signal containing the luminance signal (Y) and the color signal (C).
2. Description of the Related Art
For example, in an NTSC (National Television System Committee) color television system, a luminance signal (Y) and a color difference signal (C), which is a color signal, are contained in a composite video signal. In order to display an image by using the composite video signal, it is necessary to separate the luminance signal and the color difference signal from the composite video signal. For the separation of these signals, a decoder for composite video signals is used.
A conventional technology to perform the separation of the above two signals, that is, YC separation, is disclosed, for example, in Japanese Patent Application Laid-open Hei 8-265794. In the disclosed technology, a judgment is made as to whether there is line correlation among a plurality of neighboring signal lines including a signal line to be separated during each of horizontal scanning intervals of a composite signal. To obtain a color signal, either a trap filter adapted to allow a single predetermined frequency component to pass or a comb filter adapted to allow a plurality of frequency components to pass is selected, depending on the presence or absence of the correlation.
Therefore, prior to the selection of the filter, the correlation between the signal line to be separated and the signal line adjacent to the signal line to be separated is judged. In the above disclosed conventional technology, the correlation between these signal lines is judged not only by the use of data on color signal components of each of signal lines, but also by the use of data on color burst signals as a supplement method.
However, in the conventional technology, for example, when a composite signal indicating an image having vertical stripes not containing a color component such as a multi-burst signal is input, since the same frequency component as a color signal component contained in the composite signal and the frequency component of the neighboring line signal do not have opposite phases, the same frequency component as the color signal component is judged to have no correlation and, according to this judgement, the above trap filter is selected as the filter to obtain a color signal.
When the composite signal passes through the trap filter, the luminance signal (Y) is extracted and, at this point, the passage of a spectrum component in a high frequency band mainly including a color subcarrier related to the color signal component, for example, in the NTSC system, of a component in a frequency band having its center frequency at 3.58 MHz is inhibited. As a result, if the trap filter is used in order to separate the composite signal indicating the image having vertical stripes not containing the color component such as the multi-burst signal, the spectrum component in the band of high frequency out of a frequency component of the composite signal indicating the image having the vertical stripes is lost, thus causing a decrease in resolution of an image having stripes.
In view of the above, it is an object of the present invention to provide a decoder for composite video signals which is capable of preventing a decrease in resolution in the YC separation from the composite signal indicating an image having vertical stripes.
According to a first aspect of the present invention, there is provided a decoder for composite video signals for separating a luminance signal and a color signal from continuous composite signals including:
a correlation judging section used to judge presence or absence of line correlation among a plurality of signal lines including a signal line to be separated during each of horizontal scanning intervals of the composite signal;
a color component judging section used to judge whether a color component is contained in the composite signal;
a filter section having a first filter used to sequentially extract a color component signal contained in the composite signal of the signal line to be separated, based on a relation of the composite signal between the signal line to be separated and other signal line being adjacent to the signal line to be separated and a second filter used to sequentially extract a color component signal contained in the composite signal of the signal line to be separated; and
a selecting section used to output a selecting signal to select either of the two filters depending on judgement results from the correlation judging section and from the color component judging section to the filter section.
In the foregoing, a preferable mode is one wherein the correlation judging section judges whether there is line correlation between the signal line to be separated and the signal line being adjacent to the signal line to be separated and wherein the color component judging section serves as a stripe component judging section to judge whether an image component having stripes is contained in the composite signal to be judged.
Also, a preferable mode is one wherein the selecting section, when receiving, from the correlation judging section, a judgement result showing that there is no line correlation between the signal lines being adjacent to each other and when receiving, from the stripe component judging section, a judgement result showing that image components having vertical stripes are contained, selects the first filter, and when receiving, from the correlation judging section, the judgement result showing that there is no line correlation between the signal lines being adjacent to each other and when receiving, from the stripe component judging section, a judgement result showing that image components having vertical stripes are not contained, selects the second filter.
Also, a preferable mode is one wherein the selecting section, when receiving, from the correlation judging section, the judgement result showing that there is line correlation between the signal lines being adjacent to each other, irrespective of the judgement result from the stripe component judging section, selects the first filter.
Also, a preferable mode is one wherein the first filter is a comb filter which allows a spectrum frequency component constituting a color component signal contained in the composite signal and the second filter is a trap filter which allows a frequency component in a single frequency band containing a central frequency of the spectrum frequency component constituting the color component signal.
Also, a preferable mode is one wherein the stripe component judging section is provided with a pixel correlation judging circuit to judge whether there is correlation among neighboring pixels of the one continuous signal line to be separated.
Also, a preferable mode is one wherein the pixel correlation judging circuit is provided so as to correspond to each of a plurality of the signal lines and the stripe component judging section is provided with a processing section used to obtain a judgement result from the stripe component judging section based on the judgement results from each of the pixel correlation judging circuits and to output the judgement result as a result to be used for selection of the filter to the selecting section.
Also, a preferable mode is one wherein the pixel correlation judging circuit is provided with a plurality of time delaying devices used to obtain a plurality of continuous pixel signals of the signal line, a subtractor used to obtain a differential between both continuous pixel signals and a comparing section used to judge whether a differential obtained by the subtractor is not more than a threshold value and to output a judgement result as a result of the pixel correlation judgement to the processing section.
Also, a preferable mode is one wherein the time delaying device is made up of flip-flops and wherein the comparing section outputs, based on outputs from each of subtractors used to obtain the differential between pixel signals of lines existing before and after each of the flip-flops.
Also, a preferable mode is one wherein the stripe component judging section, when the correlation judging section has output the judgement result showing that there is line correlation among neighboring signal lines, is placed in a rest state and when the correlation judging section has output the judgement result showing there is no line correlation among neighboring signal lines, is placed in an operating state.
Also, a preferable mode is one wherein the stripe component judging section judges presence or absence of line correlation between signal lines which the one signal line to be separated is disposed between.
Also, a preferable mode is one wherein the stripe component judging section is provided with an adder used to do addition of composite signals of both the signal lines and with an substractor used to do subtraction of composite signals of both the signal lines and wherein the stripe component judging section outputs a judgement result showing whether stripe components are contained in composite signals of the signal line to be separated, based on output values from the adder and the subtractor, to the selecting section.
Also, a preferable mode is one wherein the stripe component judging section receives output values from the adder and the subtractor and is provided with a processing section which, when the output value from the adder is not less than a predetermined threshold value and the output value from the subtractor is not more than the predetermined threshold value, outputs a judgement result showing that image components having vertical stripes are contained to the selecting section.
Also, a preferable mode is one wherein the stripe component judging section, when the correlation judging section has output the judgement result showing that there is line correlation among neighboring lines, is placed in a rest state and, when the correlation judging section has output the judgement result showing that there is no line correlation among neighboring lines, is placed in an operation state.